The present invention relates to a method of producing an insulator thin film, an insulator thin film formed by the producing method, a method of manufacturing a semiconductor device using the insulator thin film, and a semiconductor device, with which it is easy to freely control the concentration gradient of a metal in a film.
Miniaturization of MOS transistors have already been coming to a gate length of 0.1 μm. The miniaturization leads to a further enhancement of operating speed of devices, a further reduction in electric power consumption, and a further reduction in the area occupied by the device. Recently, in addition, it has become possible to mount a larger number of devices per a fixed chip area, and it has therefore been realized to increase the number of functions of LSIs themselves. However, the pursuit of miniaturization is expected to be encountered by large walls, with the 0.1 μm rule as a boundary. One of the walls is the limitation in thinning the gate oxide film of a transistor. For the gate insulation film in a related-art transistor, silicon oxide (SiO2) has been used because silicon oxide can satisfy the two characteristics which are indispensable on a device operation basis, namely, the characteristic that few immobile electric charges are contained in the film and the characteristic that an interface level is little formed at the boundary between the film and silicon of the channel portion. Besides, silicon oxide has been effective also for miniaturization of devices, since a thin film of silicon oxide can be easily formed with good controllability.
However, the dielectric constant (relative permittivity) of SiO2 is as low as 3.9, so that in the transistors belonging to the generation of a gate length of 0.1 μm and the latter generations, a film thickness of 3 nm or below is required for fulfilling the transistor performances. In the case of such a film thickness, there is probably generated the problem that direct tunneling of carriers through the film would occur, resulting in an increase of leak current between the gate and the substrate.
In view of the above, a technique of forming the gate insulation film in a large thickness by use of a material higher in dielectric constant than SiO2, for preventing the tunneling phenomenon, has been studied. As the material having a higher dielectric constant, there have been investigated films of metallic oxides such as aluminum oxide (Al2O3), zirconium oxide (ZrO2), and hafnium oxide (HfO2) (see, for example, Japanese Patent Laid-open No. 2003-69011). Since these oxide films are high in dielectric constant, the film thickness of each of these oxides for obtaining a predetermined gate capacity can be several times greater than that of the film of silicon oxide, and, therefore, these oxide films are considered to be promising materials for restraining the tunneling phenomenon.
However, in the process of manufacturing a transistor using an electrode of polysilicon (poly-Si) which is currently used for silicon oxide, an activation annealing at a temperature of 1000° C. or above is required. However, the high dielectric constant films (also called high-k films) of ZrO2, HfO2 and the like are low in heat resistance, are liable to undergo crystallization, and are liable to undergo a siliciding reaction with the silicon (Si) substrate, with the result of an increase in the leak current. In an attempt to solve these problems, it has been known that the heat resistance can be enhanced and the leak current can be reduced by using Hf(Zr)SiO or Hf(Zr)SiON to which silicon (Si) and nitrogen (N) have been added (see, for example, Japanese Patent Laid-open No. 2000-58832).
A problem to be solved lies in that, in the case of forming a high-k film according to the related art, immobile electric charges are generated at the boundaries between the high-k film and the Si substrate and the poly-Si electrode, resulting in a shift of threshold voltage (Vth) and mobility degradation. Another problem to be solved lies in that, in PMOS transistors, boron with which the gate electrode is doped would, upon the subsequent heat treatment, punch through the high dielectric constant film so as to diffuse to the substrate side. It has been known that the punch-through of boron can be restrained by the addition of nitrogen. However, in the case where nitrogen is added according to the related art, nitrogen would enter into the substrate, thereby increasing the interface level.